1. Field of the Invention
The present invention relates to a helical antenna, and more particularly to a helical antenna optimal as an antenna for a mobile satellite communication terminal, a satellite-based portable terminal, or a satellite portable telephone.
2. Description of the Related Art
Conventionally, a quadrifillar helical antenna including four helical radiation elements is known as a helical antenna of this type. The quadrifillar helical antenna, however, has a problem of a complicated structure to cause susceptibility to vibration and shock.
Particularly, an antenna for use in a mobile satellite communication terminal, a satellite-based portable terminal, or a satellite portable telephone requires a wide directivity and a structure resistant to shock and vibration and suitable for placement on such a terminal or the like. It is thus difficult to use the aforementioned quadrifillar helical antenna in these communication terminals or portable telephone.
To address this, a helical antenna described in Japanese Patent Laid-open Publication No. Hei 5-206719 comprises a power distributor with a plane configuration for dividing a high-frequency signal into four and four radiation elements connected to four output terminals of the power distributor and disposed helically on the periphery of a cylindrical dielectric member such that the radiation elements are supported with a simple structure and with high rigidity.
The helical antenna described in that official gazette enables a reduced distance between each output terminal of the power distributor and a point where each radiation element is excited, and the power distributor has the plane configuration to improve impedance characteristics, thereby allowing a reduction in power supply loss.
Since the power distributor with the plane configuration can accurately control a power dividing ratio and phase differences among respective divided powers, distortion of directivity can be suppressed. In addition, if a matching element is provided at a connecting point of the power distributor to each radiation element, favorable impedance characteristics can be obtained in several percents of the frequency range to allow a reduced matching loss.
In the conventional helical antenna mentioned above, the helical antenna described in the aforementioned official gazette has the wide directivity and the structure resistant to shock and vibration and suitable for placement on a terminal or the like. The antenna is thus optimal as an antenna for a mobile satellite communication terminal, a satellite-based potable terminal, or a satellite portable telephone, but its bandwidth cannot be increased with the same size maintained.
To increase a bandwidth in such a configuration with the same size maintained, an approach has been proposed in recent years in which two different kinds of quadrifillar antennas, i.e. a total of eight radiation elements are provided together to increase the bandwidth. In this approach, however, spacings between the respective radiation elements are extremely small to increase mutual coupling. Thus, radiated radio waves are coupled to the adjacent element to reduce the radiation efficiency, or the influence of the adjacent radiation element narrows the band in frequency characteristics for input impedance to cause a reduced gain and efficiency of the antenna.
In view of the foregoing, it is an object of the present invention to provide a helical antenna capable of solving the aforementioned problems, usable at two different frequencies, and increasing its bandwidth.
A helical antenna according to the present invention comprises a radiation element helically disposed and including a first radiation element disposed on a lower portion of a dielectric member, a second radiation element disposed on an upper portion of the dielectric element, and a switching element for connection and disconnection between the first radiation element and the second radiation element.
Another helical antenna according to the present invention comprises N (N is a positive integer) sets of radiation elements, each of the radiation elements including a first conductor helically disposed on a periphery of a dielectric member in cylindrical shape, a diode for switching having one end connected to an upper end of the first conductor, and a second conductor helically disposed on the periphery of the dielectric member in cylindrical shape and connected to the other end of the diode, wherein the N sets of radiation elements are arranged in the circumferential direction of the same cylinder with the same spacings between them.
In other words, the helical antenna according to the present invention includes the switching element such as a diode interposed at some midpoint in the helical electromagnetic radiation conductor included by the helical antenna and switches the switching element, thereby changing the resonance frequency to allow the helical antenna to be used at two switched frequencies as required.
Therefore, since the helical antenna of the present invention can be used at two frequencies by switching of the diode or the like in a system which does not perform transmission and reception simultaneously, it is possible to increase a bandwidth in the helical antenna configuration with the similar size maintained.